1. Field of the Invention
Aspects of the present invention relate to a shift reactor for fuel processors, a fuel cell system employing the same, and an operating method of the same. More particularly, aspects of the present invention relate to a shift reactor for fuel processors which maintains activities of the shift catalyst without complicated supplementary devices and which remarkably shortens the startup time of the shift reactor, a fuel cell system employing the same, and an operating method of the same.
2. Description of the Related Art
A fuel cell is a type of energy generating device in which energy from a chemical reaction between hydrogen and oxygen (O2) is directly converted into electrical energy, wherein the hydrogen is contained in a fuel such as methanol, ethanol or natural gas.
Such fuel cell systems include fuel cell stacks and fuel processors (FP) as main elements and further include fuel tanks, fuel pumps, etc., as sub-elements. The fuel cell stack forms the main body of the fuel cell and is formed of a structure having a plurality of layers of unit cells, which include membrane electrode assemblies (MEA) and separators.
The fuel stored in the fuel tank is supplied into the FP by the fuel pump. The FP reforms and purifies the fuel to generate hydrogen, and supplies the generated hydrogen into the fuel cell stack. In the fuel cell stack, the supplied hydrogen electrochemically reacts with oxygen to generate electrical energy.
In the FP, a hydrocarbon is reformed in a reforming process using a catalyst. Because the catalyst is easily poisoned by sulfur compounds, the sulfur compounds that are present in the fuel must be removed before the fuel is supplied into the FP. Therefore, a desulfurization process is performed prior to the reforming process (refer to FIG. 1).
When the hydrocarbon is reformed, not only hydrogen but also carbon dioxide and carbon monoxide are generated. However, the reformed fuel should not be directly supplied into the fuel cell stack, since carbon monoxide poisons catalysts are used for electrodes of the fuel cell stack. Instead, a shift process should first be performed to remove carbon monoxide so that the concentration of carbon monoxide is reduced below 5000 ppm.
Reactions such as a shift reaction, a methanation reaction and a PROX reaction described in Reaction Schemes 1 through 3 below are conventionally used to remove carbon monoxide (CO).



In order to lower the carbon monoxide concentration to less than 5000 ppm, the temperature of the shift reactor should be 150° C. or higher. However, preheating the shift reactor to the required temperature takes about one hour. This preheating time of one hour, which is required before electrical energy can be generated, is a disadvantage of the conventional startup method of a fuel cell. Thus, there is a need to shorten the required preheating time.
U.S. Pat. No. 6,835,219 discloses a fuel processor and a method of operating the same wherein the temperature of a shift reactor is rapidly increased by adsorption heat generated when water that has not reacted in the reforming reaction is adsorbed in a water adsorbent located at the inlet side of the shift reactor. However, a disadvantage of the fuel processor and the operating method thereof is that water is not sufficiently supplied into the shift reactor and therefore, the shift reaction does not occur sufficiently.
Additionally, U.S. Pat. No. 6,838,062 discloses a method of rapidly increasing the temperature of the main elements of a fuel cell by exchanging heat using a plurality of burner systems in a fuel processing system. However, the need for additional devices and control systems results in an increase in volume and costs, which are major drawbacks.
When a fuel cell system started up, fuel is supplied into a burner of a reformer, the fuel is burned, and the reformer is thereby heated. When the temperature of the reformer reaches a predetermined point, a reforming reaction is initiated by supplying the fuel into the reformer. However, when the temperature of the shift reactor is not high enough, water vapor contained in the reformed gas can condense in the shift reactor and thereby, the shift catalysts may be deactivated. Thus, the reformed fuel may bypass the shift reactor at this step. The indirect heat transferred from the reformer and the burner increases the temperature of the shift reactor. However, a long time is required to increase the temperature of the entire shift reactor, as illustrated in FIG. 2A.
Therefore, there is a need for a simple device that can rapidly increase the temperature of the shift reactor.